# std::atof

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< cpp‎ | string‎ | byte

Defined in header `<cstdlib>` |  |
double atof( const char* str ); |  |

Interprets a floating point value in a byte string pointed to by str.

Function discards any whitespace characters (as determined by std::isspace) until first non-whitespace character is found. Then it takes as many characters as possible to form a valid floating-point representation and converts them to a floating-point value. The valid floating-point value can be one of the following:

- decimal floating-point expression. It consists of the following parts:

  - (optional) plus or minus sign
  - nonempty sequence of decimal digits optionally containing decimal-point character (as determined by the current C locale) (defines significand)
  - (optional) `e` or `E` followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent to base 10)

hexadecimal floating-point expression. It consists of the following parts: (optional) plus or minus sign `0x` or `0X` nonempty sequence of hexadecimal digits optionally containing a decimal-point character (as determined by the current C locale) (defines significand) (optional) `p` or `P` followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent to base 2) infinity expression. It consists of the following parts: (optional) plus or minus sign `INF` or `INFINITY` ignoring case not-a-number expression. It consists of the following parts: (optional) plus or minus sign `NAN` or `NAN(`char_sequence`)` ignoring case of the `NAN` part. char_sequence can only contain digits, Latin letters, and underscores. The result is a quiet NaN floating-point value. | (since C++11)

- any other expression that may be accepted by the currently installed C locale

### Parameters

str | - | pointer to the null-terminated byte string to be interpreted

### Return value

double value corresponding to the contents of str on success. If the converted value falls out of range of the return type, the return value is undefined. If no conversion can be performed, 0.0 is returned.

### Example

Run this code

    #include <cstdlib>
    #include <iostream>

    int main()
    {
        std::cout << std::atof("0.0000000123") << '\n'
                  << std::atof("0.012") << '\n'
                  << std::atof("15e16") << '\n'
                  << std::atof("-0x1afp-2") << '\n'
                  << std::atof("inF") << '\n'
                  << std::atof("Nan") << '\n'
                  << std::atof("invalid") << '\n';
    }

Output:

    1.23e-08
    0.012
    1.5e+17
    -107.75
    inf
    nan
    0

### See also

stofstodstold (C++11)(C++11)(C++11) | converts a string to a floating point value (function)
strtofstrtodstrtold | converts a byte string to a floating-point value (function)
from_chars (C++17) | converts a character sequence to an integer or floating-point value (function)
atoiatolatoll (C++11) | converts a byte string to an integer value (function)
C documentation for atof

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